Normal respiration involves the act of breathing, i.e., inhaling (inspiration) and exhaling (expiration). It is by the act of breathing that the lungs are supplied with oxygen and carbon dioxide is removed during exhalation.
During inspiration, air is inhaled into the lungs and is transferred to the blood by the gaseous exchange that occurs by the capillaries in the walls in the pulmonary alveoli. The oxygen present in the blood is utilized by the tissues resulting in the production of carbon dioxide. The carbon dioxide is in turn removed from the blood by a similar gaseous exchange that occurs at the pulmonary alveoli. During expiration, the carbon dioxide and other related pulmonary gases are removed from the body.
During normal breathing, the chest wall and abdomen both expand during inspiration and retract during expiration. At rest, inspiration lasts for about one second and expiration lasts for about four seconds. Contraction of the inspiratory muscles reduces intrapleural pressure thereby expanding the lungs and drawing air into the respiratory passages. The expiratory phase is largely passive, wherein recoil of the thoracic wall and lungs raises intrathoracic pressure to expel air.
The expansion of the thoracic cavity during inspiration is brought about by contraction of the diaphragm and intercostal muscles. The diaphragm is a modified half-dome of musculofibrous tissues separating the thorax and abdomen. The diaphragm is the chief muscle of respiration. The intercostal muscles are the inner and outer layer of muscles between the ribs. The inner layer has an expiratory function to cause exhalation while the outer layer has an inspiratory function to cause inhalation.
The muscle movements related to inspiration are generally controlled by the phrenic and intercostal nerves. The diaphragm is innervated by the left and right phrenic nerves. Spinal cord injury at the cervical and/or thoracic level can cause disruption of nerve impulses that travel from the brain to the phrenic and intercostal nerves, resulting in paralysis of the diaphragm and intercostal muscles necessitating the use of a lung assist device, such as a ventilator.
Previously, in patients suffering from respiratory muscle paralysis due to injury of the cervical spinal cord, various attempts have been made to produce artificial respiration by electrical stimulation of the phrenic nerves. In this regard, diaphragm pacing has been used successfully to restore ventilation resulting in liberation of patients from mechanical ventilation. Previous attempts have also been made to restore an effective cough by electrical stimulation techniques. For example, lower thoracic spinal cord stimulation has been successful in causing activation of the expiratory muscles and restoration of an effective cough.
Normal cough is a reflex following inspiration which includes in order: glottic closure, contraction of the expiratory muscles, and opening of the glottis. To accomplish this cycle in a patient with a diaphragm pacing system in place requires precise timing of the inspiration by the pacing system and airway occlusion at the peak of inspiration to generate the most effective cough. Further, if there is not a full inspiration to sufficiently stretch the expiratory muscles, the cough will be much less effective and will not adequately remove airway secretions or aspirated material such as food or other foreign bodies from the airway.